What is a war crime, and who decides?

What does it take for someone to officially be labelled a war criminal, and who has the power to do something about it?

Prime Minister Jacinda Ardern is the latest world leader to condemn Russia's actions in Ukraine as war crimes.

"The reports of Ukrainian civilians who had been killed, raped and severely wounded by Russian troops is beyond reprehensible. Russia must answer to the world for what they've done," she said this week.

While she stopped short of calling Vladimir Putin war criminal, other world leaders have - including United States President Joe Biden.

Ardern has described Russia's actions as "beyond reprehensible" and put forward sanctions targeting Russian oligarchs and their families in New Zealand.

Ukrainian President Volodymyr Zelensky addressed the UN days after the massacre in Bucha was exposed, labelling Putin's actions as "genocide".

Russia has denied all the accusations.

After the horrors of World War II, world leaders got together and produced the Geneva Convention - a set of rules to protect people not involved in combat, like civilians, and the doctors and nurses who treat the wounded.

International humanitarian law also prohibits mass destruction, rape, torture, and use of chemical and biological weapons.

"If the civilians are being targeted or indiscriminately being targeted, then those are what would be the most serious and the most egregious crimes under international law," International Bar Association executive director Dr Mark Ellis told Breakfast on Friday.

If someone breaks the rules, there are four ways they could get punished.

The main one is for the accused to stand trial in The Hague, where the International Criminal Court (ICC) is based.

ICC prosecutor Karim AA Khan QC announced an investigation into the Ukraine situation in February.

"I have no doubt that the International Criminal Court will also find the evidence to suggest that these crimes are being committed," Ellis told Breakfast.

But Russia, as well as other countries including the US, do not recognise the ICC's jurisdiction.

Recently the ICC condemned China's provocations toward Taiwan. But China also does not recognise the ICC's authority and has ignored the warnings.

The ICC successfully took former Yugoslav President Slobodan Milošević to trial in the Hague in 2001. He died before a verdict could be delivered, so was never punished.

The United Nations could try to prosecute, but that would mean getting the Security Council to agree - which has proved difficult in the past.

The Security Council's permanent members - China, Russia, the United States, France and the United Kingdom - hold veto powers over any decision. That power has been criticised for its undemocratic nature.

To date, almost half of the vetoes cast (146) have been by the Russian Federation or its predecessor, the Soviet Union.

The veto power is also used frequently by the US. In 1972 it cast a lone vote to block a resolution that condemned Israel for air strikes on Syria and Lebanon.

Another way in which someone could be found guilty of war crimes is if several concerned groups, like NATO and the EU, got together to create a tribunal.

"In the long term, you're able to see the possibility of bringing these individuals to justice, either in The Hague or by individual states," said Ellis.

This is what happened to the leaders of Nazi Germany, who were tried for genocide - amongst other war crimes - in 1945 and 1946.

Finally, countries which have their own war crimes legislation could try to prosecute alleged perpetrators themselves. For that to be successful, the person on trial would have to be in that country to answer for the crimes.

It is possible for any of these things to happen, but it is extremely difficult.